Transcript Atomic history powerpoint

Atomic History
How do we know all this stuff
anyway?
If atoms are too small to see…
There are between 100 billion
and 200 billion stars in our
galaxy.
About 2,500,000 (2 million
5 hundred thousand)
people live in Utah
About 6,000,000,000
(6 billion) people live on the
earth
About 300,000,000
(300 million) people live
in the USA
There are about
7,000,000,000,0
00,000
(7 quadrillion)
atoms in one
cell.
The average person is made of around
65,000,000,000,000 (65 trillion) cells
…how could we possibly know so
much about them?
• If you counted one atom
per second, it would take
you 221,968,544 years to
count all of the atoms in
one cell!
• If you took the number of
atoms that equal the
number of people on the
planet, and put them all
in a straight line, it would
be about 2 feet long.
Atoms are tiny!
• The ink dot on the
letter “i” in your
textbook contains
about
500,000,000,000
atoms.
• (That’s more than
the number of
seconds in 500,000
years!)
A Greek beginning
• The Greeks were the
first people to
propose that
everything is made
up of tiny particles
called atoms about
400 BC.
• The word “atom”
means “indivisible”
• (Aristotle disagreed
with Democritus about
atoms.)
After the Greeks
• The idea of atoms
being the building
blocks of matter
wasn’t seriously
looked at for a long
time after the Greeks
came up with the
idea.
John Dalton (1803)
• In 1803, John Dalton
reintroduced the idea
of atoms when he
proposed that all
matter is made up of
atoms.
John Dalton (1803)
• Dalton came up with
what is called the
“Atomic Theory”
• Of the 5 parts of his
atomic theory, 3 of
them are still
accepted today, and
the other two are still
considered to be
“mostly” right.
After Dalton
• In the 1850’s, people
began to see clues
that atoms might be
made up of even
smaller particles.
• We now call these
particles protons,
neutrons, and
electrons.
J. J. Thompson (1887)
• In 1887, J.J.
Thompson used what
is called a Cathode
Ray Tube to learn
about electrons.
J. J. Thompson (1887)
• By sending electricity
through a vacuum
tube, Thompson did
several experiments
through which he
found that:
– Electrons are
negatively charged
– Electrons are attracted
to magnets
– Electrons have mass
J. J. Thompson (1887)
• J.J. Thompson
came up with a
model of the atom
that is called the
“plum pudding”
model.
• He thought that
negatively charged
electrons floated
randomly around in a
positively charged
sphere.
Rutherford (1911)
• In 1911, Ernest
Rutherford did an
experiment called the
“Gold foil” experiment.
Rutherford (1911)
• Rutherford shot
positive particles at a
very thin sheet of
metal, and was able
to record where they
hit on the other side
of the metal.
Rutherford (1911)
• The expected results,
using the plum
pudding model would
look like this.
Rutherford (1911)
• Instead, Rutherford’s results
looked like this.
• Rutherford’s experiment
taught us that:
– Protons are the positive part of
an atom.
– Protons are in the center of the
nucleus and take up a very
small part of the volume of the
atom. (1/1,000,000,000,000th )
– The nucleus contains most of
the mass of the atom (99.9%).
(All of the electrons in your
whole body weigh about as
much as 4 nickels.)
– Most of the atom is empty
space.
Niels Bohr (1913)
• In 1913, Niels Bohr
developed a model of
the atom in which the
electrons move in
“orbits” around the
nucleus, similar to
how the planets orbit
around the sun.
• This is called the Bohr
model.
Our Modern Model
• Today our model of
the atom is one with
the nucleus in the
center and a “electron
cloud” around the
nucleus to show the
areas where an
electron might be
found.
Electron repulsion
• Electrons repel each other. This
is why you don’t ever REALLY
touch anything, and why you
don’t fall through the floor.
• The force of electron repulsion is
more than a trillion trillion trillion
times stronger than gravity.
• Electrons (unlike protons and
neutrons) cannot be broken
down into smaller parts.
Problems with using a model atom
• There are a few
limitations (problems)
that occur anytime
you try to make a
model of an atom.
Problems with using a model atom
• 1- You can’t accurately
show the distance between
particles in the atom.
– If the nucleus were the size of
a bean, the atom would be the
size of a football stadium.
– If the nucleus were the size of
a tennis ball, the electrons
would reach 4 miles out.
– If the nucleus were the size of
a basketball in the center of
the Earth, the electrons would
be cherry pits flying around in
the outer atmosphere.
Problems with using a model atom
• The motion of electrons
cannot be accurately
shown in most models.
– Electrons do not move
in nice circular orbits
around the nucleus, but
that is how most models
have to show them,
because they can’t show
the electron movement.
– (Electrons move so fast
that they could circle the
Earth in 18 seconds).
Electron motion video clip
Future Models
• Our current model of
the atom will probably
change as we learn
more, experiment
more, and build on
the knowledge of
those who came
before.
• Atomic movie